Aluminum dialkylphosphinate and zinc dialkylphosphinate are halogen-free flame retardants with good thermal stability and flame resistance, and now have been widely used to prevent polyamides and polyesters from flaming as an alternative to brominated flame retardants. They can be prepared by various methods.
U.S. Pat. Nos. 6,300,516 and 6,355,832 describe a method for preparing aluminum dialkylphosphinate by feeding phosphinic acid or alkali metal salts thereof into a kettle at one time and reacting the phosphinic acid or alkali metal salts thereof with alkenes in the presence of azo initiators to prepare the intermediate dialkyl phosphinic acids or alkali metal salts thereof, the products obtained were then mixed with aluminum salt to produce aluminum dialkylphosphinate. The intermediate dialkylphosphinic acids/alkali metal salts thereof prepared by the method above contain a large amount of the long chain alkyl phosphinates/alkali metal salts, wherein the contents of which are between 7% and 12%. Physical properties of the engineering plastics will be affected as a result of containing too much long chain alkyl phosphinate, and the the product quality control will be affected if the contents of the long chain alkyl phosphinic acids/alkali metal salts vary in a large range. Therefore, it is necessary to control the contents of the long chain alkyl phosphinate to be less than 10% and to keep it stable.
U.S. Pat. No. 7,635,785 describes a method for preparing dialkylphosphinate by, using water as the co-solvent, feeding phosphinate into a kettle at one time and reacting phosphinate with alkene in the presence of an initiator to prepare dialkylphosphinate. The method described above can effectively control the contents of the long chain alkyl phosphinate to be less than 6%. However, due to the solubility of ethylene in water is very low, a high pressure has to be used in this method (20 bar) in order to achieve a satisfactory yield. The use of high pressure condition increases the risk of the reaction, therefore this method is not suitable for industrial production.
Chinese Patent CN101891762 describes a method for preparing sodium diethylphosphinate by adding sodium phosphinate into a kettle at one time and reacting the sodium phosphinate with ethylene in the presence of photoinitiators to produce sodium diethylphosphinate. Since the sodium diethylphosphinate prepared by the method above contains 2.5% mole or more of sodium monoethylphosphinate, aluminum diethylphosphinate transformed from the sodium diethylphosphinate is inevitably doped with a high level of aluminum monoethylphosphinate which has lower thermal stability, thereby limiting the application of the products in factory plastics.
Chinese Patent CN103319524 describes a method for preparing dialkylphosphinate by feeding sodium phosphinate into a kettle at one time and reacting the sodium phosphinate with alkene in the presence of metal complexes and initiators to produce dialkylphosphinate. Although the method above avoids the production of monoalkyl phosphinate, a variety of organic solvents are used in order to stabilize the transition metal complexes, which adversely affects the purification of the product and the environment.
Chinese Patent CN103172670 describes a method for directly preparing aluminum diethylphosphinate by, using water as solvent, feeding aluminum phosphinate into the kettle at one time and reacting the aluminum phosphinate with ethylene in the presence of initiators to produce aluminum diethylphosphinate directly. Because the method above requires reactions at high pressure, there are safety risks, which is not suitable for industrial production.
So far, all methods which have been reported for preparing dialkyl phosphinic acid or salts thereof involve feeding phosphinic acid/salts thereof into a reaction kettle at one time. All these methods need to be improved, as all of them have drawbacks in controlling the contents of long chain alkyl phosphinates and monoalkyl phosphinates.